1. Field of the Invention
The present invention relates to a technology of smoothing the arrangement of lattice points when a profile is created.
2. Description of Related Art
Imaging devices such as displays and printers normally use color image data whose pixel colors are tonally rendered with specific color components. Image data defines colors using various color spaces such as RGB and CMY-based color spaces, for example. The RGB color space uses three colors R (red), G (green), and B (blue). The CMY-based color space (including light cyan lc, light magenta lm, dark yellow DY, and black K) uses C (cyan), M (magenta), and Y (yellow). These colors are generally specific to imaging devices. A color correction LUT (lookup table) is used to define correspondence between colors for devices so that various imaging devices can output the same image in same colors.
In consideration for saving the storage capacity or improving the workability to create the color correction LUT, it is impracticable for the color correction LUT to define the correspondence between all colors the imaging devices can output. Generally, the color correction LUT defines the correspondence between a specified number of representative colors. An interpolation operation is used to calculate the correspondence for the other colors. That is, no color measurement is performed for a large number of colors. The color measurement is performed by outputting colors from an imaging device within a range capable of actual color measurement to prescribe the color correction LUT for a specified number of representative colors.
The interpolation operation accuracy depends on the arrangement of lattice points formed in a color space using representative colors defined in the color correction LUT. That is, the interpolation operation generates information about a lattice point to be interpolated from information about a lattice point near that lattice point. When the lattice point arrangement is unsmooth, the interpolation accuracy degrades to increase the possibility of generating a tone jump or a color distortion. To solve this problem, there is known the technology of smoothing the lattice point arrangement (e.g., JP-A No. 116012/2003). Conventionally, lattice points are positioned in the RGB color space or the CMY-based color space. A three-dimensional filter is used to smooth the relationship between a given lattice point and surrounding ones.
The above-mentioned conventional smoothing technology maintains the color gamut or constrains a specific lattice point to a specific position before and after smoothing. As a whole, it has been difficult to obtain smooth lattice points. To ensure as wide a color gamut as possible in the color correction LUT, it is necessary to not only maintain lattice points positioned to the color gamut boundary, but also smooth the arrangement. To maintain a specific color such as gray, it is necessary to not only arrange lattice points on the gray axis, but also smooth the relationship with the surrounding lattice points. While the conventional technology takes a weighted average of the smoothed state and the unsmoothed state, there is not necessarily a relationship between the weighted average and the smoothed ideal arrangement. It has been impossible to smooth the lattice point arrangement by providing detail conditions such as maintaining the color gamut and fixing an intended lattice point to an intended color as mentioned above.